No, 20gpXCLXSL is probably too high.
Replacing the spellcasting requirement shouldn't be too expensive if you want it to ever be used. The real cost will come from the substitution for XP.
Now that I think of it, though, 5GP per XP is probably too little, as it lets you craft an item at 70% of its price (assuming you have the necessary feats and spells.) 15GP per XP probably makes more sense, as then crafting an item by buying dust costs 110% of its price (the extra is worth it to some people because you can customize what you want), and crafting by dusting items instead of selling them costs 80% of the price.